External pressure metal corrugated expansion joint, to put it bluntly, is a compensator that makes the bellows bear the medium pressure outside. You may ask: Why should good bellows be left outside? This has to start with its "arch-enemy"-internal pressure.
What is the difference between external pressure and internal pressure structure? Looking at the original intention of design from the angle of force
Internal pressure expansion joint, the bellows is directly soaked in the pipeline medium, and the pressure is pushed from the inside to the outside. At this time, the bellows wall is subjected to circumferential tensile stress, just like blowing air into a balloon, and the peaks and troughs are in tension state. The external pressure type is just the opposite-the medium pressure acts on the outer surface of the bellows, and the inside of the bellows is actually normal pressure or low pressure. And guess what? In this way, the bellows wall is mainly subjected to compressive stress, not tensile stress.
For high temperature and high pressure steam pipelines, internal pressure bellows are easily scrapped in advance because of fatigue cracks (tensile stress is an accelerating factor). The external pressure type moves the bellows between the outer sleeve and the end pipe, and the medium goes from the inside. The bellows "blows cold air" outside (actually, it is also insulated), and the stress state becomes pressurized. The compression resistance of stainless steel corrugated pipe is much stronger than the tensile resistance, so the external pressure type can withstand higher pressure and temperature under the same working conditions. This is not something to think about, but a pile of decades of engineering data.
The core components of external pressure single axial expansion joint: bellows, end tube, guide tube and outer sleeve
Let's take one apartExternal pressure single axial expansion joint(This thing is in the product information of this site), and see what it looks like inside.
- Bellows: Core elastic element, typically multilayered thin wall pressed with austenitic stainless steel (304 or 316L). Because it is an external pressure structure, the number of layers can be made less than that of the internal pressure type, so that the stiffness is reduced and the compensation amount is greater.
- End tube: The two sections of pipe connecting the pipe, divided into the left end pipe and the right end pipe. One end is fixed, and the other end moves with the thermal displacement of the pipe.
- guide tube: It is installed inside the medium flow channel to protect the inner wall of the bellows from being washed by high-speed fluid. Here is a key: the direction of the arrow of the guide tube must point to the flow direction of the medium, and if it is installed backwards, it is equivalent to not installed. Regarding this, the question and answer of this site "The specific function of the expansion joint guide tube" has a detailed explanation.
- Outer sleeve: External pressure type unique "armor". A circle of thick tubes is sleeved outside the bellows, and the medium pressure acts on the outer surface of the bellows through the outer sleeve. The outer sleeve also plays a supportive and protective role to prevent the bellows from being damaged by external forces.
To put it bluntly, the external pressure single axial type means that the medium takes the "inner channel" and the bellows takes the "outer channel", which does not directly contact each other, but the pressure is transmitted to the bellows through the outer sleeve. The structure is not complicated, but if you want to design it stable and air-free, there are many doorways.
What scenarios is this thing mainly used in? Power station, cement, steam pipeline... Don't choose the wrong model
You thought it was a universal part? Wrong. External pressure metal corrugated expansion joint has its "home field".
The most classic ones are the main steam pipeline and reheating steam pipeline of the power station. The pressure of these pipes is above 10 MPa and the temperature is up to 600 °C. Under this working condition, the life of the internal pressure type may only be two or three years, and it can easily run for ten years if it is replaced with the external pressure type. The reason just mentioned-compressive stress conditions are more resistant to fatigue.
What about the cement industry? The high-temperature fan outlet and the flue gas pipeline at the head and tail of the kiln have large temperature fluctuations and dust. The external pressure type expansion joint can be made into wear-resistant lining because the guide tubeMetal Corrugated Expansion Joints in Cement IndustryDesign experience, can withstand abrasion. However, note: if the medium contains sulfur oxides (such as wet flue gas after desulfurization), you have to consider corrosion-resistant materials or PTFE lining. At this time, external pressure type is not necessarily cost-effective.
Air cooled island vacuum pipe. There is a "double hinge expansion joint of air-cooled island vacuum pipeline" in this product, which is a special structure. If it is only a simple axial displacement, the external pressure single axial type is also a candidate-because the external pressure bellows bears external compressive stress in vacuum state, and its stability is better than that of the internal pressure type.
Find out whether the medium pressure in your pipe is pushing from the inside to the outside or from the outside to the inside. The pressure of the steam pipe comes from the inside, and the external pressure type is used; The pressure of the water pipe is also internal, so it is not wrong to use the external pressure type, but the cost is high. It is more economical to use the general corrugated expansion joint in low pressure and normal temperature applications.
Three hard indexes in model selection: pressure grade, compensation amount and stability check
Choose external pressure expansion joint, not just look at the caliber to place an order. Three hard indicators, one less is waiting for rework:
First, the pressure rating.The nominal pressure of the external pressure type depends not on how much internal pressure the bellows can withstand, but on how much pressure the sealing structure between the outer sleeve and the end tube can carry. Commonly used grades PN6, PN10, PN16, PN25. If the design pressure of the pipeline is 2.5MPa, choose PN25, but don't forget to check the critical pressure of instability of the bellows under external pressure. This calculation is mentioned in the "Stiffness and Calculation Formula of Bellows" in our station. Simply put, the wall thickness, wave height and wave pitch of the bellows determine whether it can maintain its shape under external pressure. If the pressure rating is chosen low, the bellows will crush like a flattened can.
Second, the amount of compensation.External pressure single type axial type mainly absorbs axial displacement. The amount of compensation depends on the wavenumber of the bellows. The compensation capacity of each wave is generally 10~25mm (related to the diameter and wave height). For example, the thermal elongation of the pipe is 80mm, which requires at least 4 waves. However, with more wavenumbers, the overall length increases, and the risk of instability also increases. Therefore, don't blindly add waves, and use duplex structures or tie rods to limit lateral displacement when necessary.
Third, stability check.This is a pit unique to the external pressure type. The outer surface of the bellows is surrounded by the outer sleeve, and if the gap between the inner diameter of the outer sleeve and the outer diameter of the bellows is too large, the bellows will develop "circumferential instability"-the crest collapses inward. Solution: Control the clearance to not exceed 5 times the wall thickness of the bellows, or add a guide ring. Many manufacturers are lazy in this link, and as a result, they are leaked after six months of operation.
Frequently stepped pits in installation and operation and maintenance: tie rod adjustment, guide tube direction, pre-deformation treatment
No matter how good things are, it will be useless if you install them wrong. Let's talk about a few real rollover cases:
Tie rod adjustment.External pressure single type axial type usually with tie rod (this site FAQ "Expansion joint tie rod function" has an explanation). The role of the tie rod is to fix the bellows length during transportation and installation, preventing it from expanding and contracting freely. However, many construction teams directly weld or screw the tie rod to death, forgetting to loosen the nut before the pipeline is put into operation. As a result, as soon as the pipeline heated up, the expansion joint did not move at all, and the bellows was forcibly stretched to plastic deformation, which leaked the next day. Correct practice: After installation and before pressure test, adjust the tie rod nut to the reserved compensation amount position (such as pre-tension amount), and then lock the back cap. For the specific adjustment method, see "How to adjust the expansion joint tie rod nut".
Direction of the guide tube.The arrow points to the direction of the media flow, a cliche that somebody commits every year. When the guide tube is installed backwards, the medium directly washes the root of the bellows, and the wear speed is doubled. Especially the dusty airflow, which can be worn out in two months.
Pre-deformation treatment.When steam pipeline is installed in cold state, the expansion joint needs to be pre-stretched (compressed) to offset the displacement difference from cold state to hot state. The external pressure expansion joint itself is longer than the internal pressure type, and it is easy to press the bellows into a "bamboo shape" during pre-stretching. Experience: Calculate the thermal elongation first, then pre-stretch (or pre-compress) according to the compensation amount of 60% ~70%, and then lock the tie rod. Release slowly after commissioning.
Alas, two days ago, I met a customer. The steam pipeline used an external pressure expansion joint. As a result, the guide tube was installed in the reverse direction, and the tie rod was not loose. It exploded in a month. Lost hundreds of thousands. You say it was wrong or not? Therefore, structural selection is technology, installation, operation and maintenance is conscience, and both are indispensable.